1. Field of the Invention
The field of the invention relates to a method for determining the change in position of an object in an item of luggage by means of two X-ray images.
2. Description of Related Art
X-ray examination installations are known that can use appropriate methods to ascertain the change in position of an item of luggage between a first X-ray picture (which has usually been taken in another luggage-examination installation) and a second X-ray image. An effect of these methods is that, in the second-stage apparatus, only the suspect regions identified in the first-stage apparatus still need be examined to see whether they contain dangerous items or substances. Consequently, significant time savings and a much higher throughput rate are achieved. The first stage and second stage apparatuses can be physically separated. Unlike other systems, there may be two different examination apparatuses in different locations. Nor is any assumption made about the transportation of the items of luggage.
The basis of the known methods is that an image segmentation is carried out within the suspect region and the segmented objects ascertained in the second X-ray image. But such methods often fail due to the distorting and transparent structure of the X-ray images. If the two imaging systems also differ from each other in terms of their geometry the difficulties are even greater, as in that case, even with a strong absorber, the amplitude values in the images do not match. Added to this are also distortions in the image which stem from different perspectives. This means that even the topology of the images of the objects is not necessarily retained. Thus for example a cylinder can be displayed in one image as a rectangular structure, but in a second image as a round structure. Even in cases where the change in position of the item of luggage is correctly determined and thus the suspect region ascertained in the first X-ray image is precisely determined in the second X-ray image, there are still problems if the suspect object has moved before the second X-ray image is taken. In this case, the whole item of luggage must once again be examined fully, which results in a reduction of the throughput rate.